1. Field of the Invention
This invention relates to a method and an apparatus for maintaining the level of a molten material in a glass manufacturing system, and in particular, for controlling the feed and melting rates of a precursor material as a function of the level of the molten material.
2. Technical Background
In a typical glass manufacturing system, various raw constituent or batch materials, generally in a granular state, are introduced, or “charged” into a melting furnace. The batch materials are melted to form a viscous molten material that can be flowed to a fabrication portion of the system. The viscous molten material, when cooled, forms a glass. For the purposes of discussion and not limitation, the viscous molten material will hereinafter be referred to as molten glass or glass melt.
It is desirable during the course of the melting process that the level of the glass melt in the melting furnace is maintained at a consistent level. If the level of the melt fluctuates excessively, the glass melt can “wash” different areas of the walls of the melting furnace. This is a reasonable concern because the walls of the furnace are generally lined with a refractory brick that dissolves into the melt over a period of time, and thus variations in composition between different areas of the refractory bricks forming the walls may be reflected in the melt. Additionally, various melting by-products may accumulate on the refractory walls over the course of a melting campaign. For example, the surface of the melt is generally covered with unmelted batch materials, and foam resulting from the melting. A fluctuating level may result in an inconsistent chemical composition of the melt, or may incorporate solid refractory and/or batch inclusions in the melt. Finally, a fluctuating level, and the attempt to compensate, can lead to thermal instability of the melt.
Historically, a significant amount of glass manufacturing for glass sheets has been performed in a float system, wherein the precursor materials are first melted in a melting furnace, fined to remove gaseous inclusions, and flowed onto the surface of a second molten medium, typically tin.
More recently, a fusion process has been used to produce exceptionally defect free glass sheets, useful for manufacturing optical displays, wherein the precursor materials are first melted in a melting furnace, then flowed through a system of tubes or pipes and vessels to a forming pipe. The forming pipe comprises an open-top vessel that includes converging forming surfaces. The molten glass overflows the top of the vessel and flows down both sides of the forming vessel, including the converging forming surfaces. The separate flows then rejoin at the line where the converging forming surfaces meet. Thus, molten glass that has been in contact with the refractory surface of the forming pipe is joined within the interior of the formed sheet, wherein the exterior surfaces of the sheet have not been contacted by the forming surfaces. The tubes and vessels between the melting furnace and the forming pipe are typically formed of a refractory metal, such as platinum or a platinum-rhodium alloy, and are collectively referred to as the platinum system.
Pressure drops that occur through the platinum system may vary due to temperature fluctuations within the platinum system, and result in level fluctuations that propagate back to the melting furnace, making fusion-type glass making processes more prone to level fluctuations than other processes. Generally, measuring the level of the glass in the melting furnace directly is difficult due to the aforementioned melting by-products: the molten glass in the melting furnace may not present a well-defined surface. Thus, indirect methods may used. Typically, level control is implemented by varying the rate of batch material feed to the furnace. However, a typical screw-type batch feeder is generally a high capacity apparatus that lacks the fine control necessary to maintain adequate level control without overshooting the target level, thereby creating additional level fluctuations.